1. Field of the Invention
The present invention generally relates to a digital transmitter identification information signal (TII signal) detection circuit and the method thereof, and more particularly, to a circuit and a method for detecting TII signal in a digital audio broadcast (DAB) system.
2. Description of Related Art
A DAB system is a broadcast system with high transmission quality, where the system adopts OFDM (Orthogonal Frequency Division Modulation) scheme so as to possess good robustness against channel decay and noise.
In the DAB standard, a TII signal is carried by an OFDM symbol, and the TII signal is defined at a position of a null symbol. The TII signal provides the transmitter identification information, so that a receiver is able to recognize where the data sent by a transmitter comes from and the zone to which the transmitter belongs. A TII is mainly categorized into main identifier (MainId) and sub-identifier (SubId). In the DAB system, the TII signal is presented once every two adjacent OFDM frames and takes a position of null symbol.
The MainId has seven bits with a coding range of 0-69, while the SubId has five bits with a coding range of 1-23. In the DAB standard, the definitions of the MainId and SubId are specified, and a TII signal is corresponding to the coding values of a MainId and a SubId, namely, the matching relations of MainId-SubId-TII signal are specified in the DAB standard.
According to the DAB standard, a TII signal in an OFDM symbol is conveyed by a couple of pairs of adjacent sub-carrier signals. As mentioned above, a TII signal can be looked up by the coding values of the corresponding pair of MainId and SubId. In other words, once a receiver decodes an implicated TII signal, the coding values of MainId and SubId with the transmitter to locate are accordingly solved.
Assuming the coding values of MainId and SubId are respectively coding p and c, the TII signal STII(t) can be expressed according to the DAB standard by the following equation:
                    S        TH            ⁡              (        t        )              =          Re      ⁢              {                              exp            ⁡                          (                              j                ⁢                                                                  ⁢                2                ⁢                π                ⁢                                                                  ⁢                                  f                  c                                ⁢                t                            )                                ⁢                                    ∑                              m                =                                  -                  ∞                                            ∞                        ⁢                                                  ⁢                                          ∑                                  k                  =                                                            -                      K                                        /                    2                                                                    K                  /                  2                                            ⁢                                                          ⁢                                                Z                                      m                    ,                    0                    ,                    k                                                  ⁢                                                      g                                          TH                      ,                      k                                                        ⁡                                      (                                          t                      -                                              mT                        F                                                              )                                                                                      }                                g                  TH          ,          k                    ⁡              (        t        )              =                  exp        ⁡                  (                      j            ⁢                                                  ⁢            2            ⁢            π            ⁢                                                  ⁢                                          k                ⁡                                  (                                      t                    -                                          T                      NULL                                        +                                          T                      U                                                        )                                            /                              T                U                                              )                    ⁢      Re      ⁢                          ⁢              ct        ⁡                  (                      t            /                          T              NULL                                )                    wherein TU represents the reciprocal of the frequency difference of two adjacent sub-carriers, TNULL represents the signal duration of a null symbol, fc represents the center frequency of the DAB signal, Zm,0,k represents the complex number value carried by the k-th sub-carrier in the null symbol, Re{●} represents an operator for extracting real number value, exp(●) represents exponent function and Rect(●) represents square wave function. When a TII signal is not transmitted, Zm,0,k is equal to zero; when the TII signal is transmitted, Zm,0,k is decided by the coding value p of MainId and the coding value c of SubId.
According to the DAB standard, Zm,0,k is expressed by the following equation:Zm,0,k=Ac,p(k)exp(jφk)+Ac,p(k−1)exp(jφk−1)wherein φk and φk−1 represent phase reference symbol (PRS) defined in the DAB standard, and Ac,p(k) represents amplitude which are respectively defined by different equations in the four transmission modes I, II, III and IV of the DAB standard. For the transmission modes II, Ac,p(k) is defined as follows:
                    A                  c          ,          p                    ⁡              (        k        )              =                            ∑                      b            =            0                    3                ⁢                                  ⁢                              δ            ⁡                          (                              k                ,                                                      -                    192                                    +                                      2                    ⁢                    c                                    +                                      48                    ⁢                    b                                                              )                                ⁢                                    a              b                        ⁡                          (              p              )                                          +                        ∑                      b            =            4                    7                ⁢                                  ⁢                              δ            ⁡                          (                              k                ,                                                      -                    191                                    +                                      2                    ⁢                    c                                    +                                      48                    ⁢                    b                                                              )                                ⁢                                    a              b                        ⁡                          (              p              )                                            and                    A                  c          ,          p                    ⁡              (        k        )              =                            0          ⁢                                          ⁢          if          ⁢                                          ⁢          k                <                              -            192                    ⁢                                          ⁢          or          ⁢                                          ⁢          k                >                  192          ⁢                                          ⁢          or          ⁢                                          ⁢          k                    =      0                  δ      ⁡              (                  i          ,          j                )              =          {                                    1                                                              if                ⁢                                                                  ⁢                i                            =              j                                                            0                                                              if                ⁢                                                                  ⁢                i                            ≠              j                                          wherein ab(p) is calculation factor defined in the lookup table of FIG. 1.
FIG. 1 is a lookup table (LUT) showing the relationship between the coding value p of MainId and the calculation factor ab(P) of TII signal according to the DAB standard, and FIG. 2 is a signal wave diagram of a TII signal of transmission mode II. Referring to FIGS. 2 and 1, the TII signal thereof is corresponding to a coding value c=16 of SubId and a coding value p=4 of MainId, and it can be found according to the LUT of FIG. 1 that ab(P)=00011110. Then, Ac,p(k) and Ac,p(k−1) can be calculated respectively by:Ac,p(k)=δ(k,−16)+δ(k,33)+δ(k,81)+δ(k,129)Ac,p(k−1)=δ(k−1,−16)+δ(k−1,33)+δ(k−1,81)+δ(k−1,129)
It can be seen from the above-mentioned calculation results that Zm,0,k is not equal to zero only when k=−16, −15, 33, 34, 81, 82 and 129, which means the TII signal is not equal to zero only corresponding to the sub-carrier of the TII signal, wherein the indices of the sub-carriers take −16, −15, 33, 34, 81, 82 and 129. Besides, as described above, the TII signal obtained by calculation has a characteristic that every two adjacent sub-carriers with signal values (non-zero) form a pair. In other words, the TII signal has a couple of pairs of two adjacent sub-carrier signals.
F. Van de Laar, N. Philips and J. Huisken have pointed out in their paper “Towards the Next Generation of DAB Receivers, EBU Technical Review, Summer 1997” that every pair of sub-carrier signals in a TII signal is the same as the sub-carrier signals of the reference symbol, so that the TII signal can be obtained by conducting a checking operation on the sub-carrier signals of null symbols. In addition, three TII signals can be solved every time according to several OFDM frames.
However, the above-mentioned scheme needs to check the sub-carrier signals of every null symbol to obtain TII signals, which accordingly needs a register with large capacity to register the values of every sub-carrier signal. Furthermore, the procedure to check every sub-carrier of the null symbols consumes a great amount of time. In short, the conventional TII signal detection method takes too long time and needs a register with large capacity to register sub-carrier signals, therefore, the prior art does not meet the requirement of real-time network.